Systems and methods to determine relative elemental concentrations from nuclear spectroscopy measurements

Abstract

A system may include a pulsed neutron generator designed to emit neutrons into a borehole of a geological formation using a pulsing scheme. The system may also include a gamma-ray detector designed to take measurements of capture gamma-rays during a time period during the pulsing scheme. The system may also include data processing circuitry designed to calculate one or more sigma values based at least in part on the measurements of the capture gamma-rays taken during the time period during the pulsing scheme. The data processing circuitry may also calculate a factor of yields to weights value based at least in part on the one or more sigma values and convert a plurality of relative yields of corresponding elements in the geological formation to a plurality of elemental relative weights based at least in part on the factor of yields to weights.

The invention claimed is: 
     
       1. A system comprising:
 a pulsed neutron generator configured to emit neutrons into a borehole of a geological formation using a pulsing scheme; 
 a gamma-ray detector configured to take measurements of capture gamma-rays during a time period during the pulsing scheme; and 
 data processing circuitry configured to calculate one or more sigma values based at least in part on the measurements of capture gamma-rays taken during the time period during the pulsing scheme, to calculate a factor of yields to weights value based at least in part on the one or more sigma values, and to convert a plurality of relative yields of corresponding elements in the geological formation to a plurality of elemental relative weights based at least in part on the factor of yields to weights. 
 
     
     
       2. The system of  claim 1 , wherein the data processing circuitry is configured to estimate a property of the geological formation based at least in part on the plurality of elemental relative weights in the geological formation. 
     
     
       3. The system of  claim 1 , wherein the one or more sigma values are calculated via the data processing circuitry based at least in part on a decay rate of the measurements of capture gamma-rays. 
     
     
       4. The system of  claim 1 , wherein gamma-ray detector is configured to take measurements of the capture gamma-rays during the time period, wherein the time period is during an off period of the pulsing scheme. 
     
     
       5. The system of  claim 4 , wherein the time period occurs sufficiently after the pulsed neutron generator has turned off to cause the gamma-ray detector to detect substantially no photons due to inelastic scattering. 
     
     
       6. The system of  claim 4 , wherein the time period comprises the time from 10 micro seconds (μs) into the off period to 480 μs into the off period. 
     
     
       7. The system of  claim 1 , wherein the one or more sigma values comprises an apparent sigma, calculated via the data processing circuitry, that balances a borehole sigma and a formation sigma. 
     
     
       8. The system of  claim 1 , wherein the one or more sigma values comprises a plurality of sigma values, calculated via the data processing circuitry, which together balance borehole effects and formation effects. 
     
     
       9. The system of  claim 1 , wherein the borehole of the geological formation is cased. 
     
     
       10. The system of  claim 1 , wherein the factor of yields to weights value, calculated via the data processing circuitry, does not ignore hydrogen or chlorine in the borehole or the geological formation. 
     
     
       11. A method for determining a factor of yields to weights value in a borehole of a geological formation comprising:
 emitting neutrons, from a neutron generator, placed into the borehole in the geological formation, to cause capture and inelastic scattering events that generate photons; 
 detecting the photons using one or more detectors; 
 determining, via a processor, from the detected photons, at least one of:
 a ratio of photon counts between two detectors; or 
 an apparent sigma; 
 
 determining, via the processor, the factor of yields to weights value based at least in part on the at least one of:
 the ratio of photon counts between two detectors; or 
 the apparent sigma; 
 
 converting, via the processor, a plurality of relative yields of elements in the geological formation to a plurality of elemental concentrations using the factor of yields to weights value; and 
 determining, via the processor, a property of the geological formation using at least one of the plurality of elemental concentrations. 
 
     
     
       12. The method of  claim 11 , wherein the photons are detected during a time period defined by timing gates, during a pulsing scheme of the neutron generator, operatively controlled by the processor. 
     
     
       13. The method of  claim 11 , wherein determining the factor of yields to weights value takes into account elements in porous space. 
     
     
       14. The method of  claim 13 , wherein the elements in porous space include at least hydrogen and chlorine. 
     
     
       15. A method comprising:
 emitting neutrons, out of a downhole tool placed into a borehole in a geological formation, to cause inelastic scattering events that generate photons; 
 detecting the photons using a plurality of detectors; 
 determining, via a processor, a count rate ratio of the photons between at least two of the plurality of detectors; 
 determining, via the processor, a factor of yields to weights value based at least in part on the count rate ratio; 
 converting, via the processor, a plurality of relative yields of corresponding elements in the geological formation to a plurality of elemental relative weights using the factor of yields to weights value; and 
 determining, via the processor, a property of the geological formation using at least one of the plurality of elemental relative weights. 
 
     
     
       16. The method of  claim 15 , wherein the neutrons are emitted via a pulsed neutron generator. 
     
     
       17. The method of  claim 16 , wherein the photons are detected while the pulsed neutron generator is on. 
     
     
       18. The method of  claim 15 , wherein a first detector of the at least two of the plurality of detectors is positioned closer to a neutron source emitting the neutrons than a second of the at least two of the plurality of detectors. 
     
     
       19. The method of  claim 15 , wherein determining the factor of yields to weights value comprises using the count rate ratio and at least one of the plurality of relative yields of corresponding elements. 
     
     
       20. The method of  claim 19 , wherein the at least one of the plurality of relative yields of corresponding elements comprises an inelastic iron relative yield.